PLANT DISEASES 



DISEASES NOT DUE TO PARASITES 

 OR OF UNKNOWN ORIGIN. 



1225 - Mottle-Leaf of Citrus Trees in Relation to Soil Conditions. — i^ymax, j., Briggs 



J. C. A., and Mc I,ane J. W.,in Journal of Agricultural Research, Vol. VI, No 19, pp. 721, 

 739, PI. XCVII. Washington, D. C, 1916. 



Mottle-leaf of Citrus trees is characterised by the disappearance of 

 the chlorophyll from parts of the leaf beginning at the points which are 

 furthest away from the mid-rib and the larger veins. As the alteration 

 progresses the yellowish spots increase in size, and finallj^ unite, until the 

 only remaining chlorophyll is confined to narrow areas along the veins. 

 In advanced stages there is a marked decrease in the size, quality and 

 yield of the fruit. No organism has as yet proved to be associated with 

 this condition, but it is generally stated that the nematode Tylenchulus 

 semipenetrans Cobb (i) is widely distributed in districts infected with 

 mottle-lead. 



Mottle-leaf is found in most Citrus fruit plantations in California, 

 but it is more prevalent in some districts than in others. All the Citrus 

 fruits grown in California are affected, including the Washington Navel, 

 Thompson Improved Navel, and Valencia orange, grapefruit, tangerine 

 and lemon. 



Other conditions being equal, the buds grafted on to C. aitrantium 

 are more susceptible than those grafted on to C. sinensis ; in a mixed 

 grove of Washington Navel and Valencia oranges both varieties were 

 equally attacked. 



Orchards manured with organic substances, such as stable manure 

 or clover crops, usually did better than those treated v/ith chemical ferti- 

 lisers. In the latter case the changes induced are always more extensive 

 than when nitrate of soda alone is employed. 



The experiments in question indicate that the origin of the disease 

 must be sought in the conditions of growth and more especially in the 



(i) See B. July IQ15, No. 77^- {Ed.). 



